Process for producing carbon fibres

ABSTRACT

CARBON FIBERS ARE PRODUCED BY HEAT-TREATING POLYACRYLONITRILE FIBRES IN AN OXIDIZING ATMOSPHERE AT A TEMPERATURE BETWEEN 150*C. TO 400*C. FOLLOWED BY CARBONISING AT A TEMPERATURE OF OVER 400*C. IN A CARBONISING ATMOSPHERE CONTAINING 0.5-20% BY VOLUME WATER VAPOUR. IN A PREFERRED EMBODIMENT, WATER VAPOUR IS PROVIDED TO THE CARBONISING ATMOSPHERE BY PASSING THE FIBROUS MATERIAL THROUGH WATER OR A LIQUID CONTAINING WATER BETWEEN THE OXIDATIVE HEAT TREATMENT AND THE CARBONISATION.

0d. 30, 1973 R E E HAL 3,769,390

PROCESS FOR PRODUCING CARBON FIBRES Filed March 11, 1971 INVENTOR RCLMND WEI SEE-50K, CARLHANS SULING, GOTTFRIED PAMYUS, LOTHAR PREIS. v

United States Patent 3,769,390 PROCESS FOR PRODUCING CARBON FIBRES Roland Weisbeck, Odenthal-Voiswinkel, Gottfried Pampus, Leverkusen, Carlhans Suling, Odenthal-Hahnenberg, and Lotliar Prets, Cologne, Germany, assignors to Bayer Aktiengesellschaft, Leverkusen, Germany Filed Mar. 11, 1971, Ser. No. 123,368 Claims priority, application Germany, Mar. 14, 1970,

P 20 12 284.3 Int. Cl. C01b 31/07 US. Cl. 423-447 16 Claims ABSTRACT OF THE DISCLOSURE Carbon fibres are produced by heat-treating polyacrylonitrile fibres in an oxidizing atmosphere at a temperature between 150 C. to 400 C. followed by carbonising at a temperature of over 400 C. in a carbonising atmosphere containing 0.5-20% by volume water vapour. In a preferred embodiment, water vapour is provided to the carbonising atmosphere by passing the fibrous material through water or a liquid containing water between the oxidative heat treatment and the carbonisation.

The invention relates to a process for producing thin carbon fibres, such as for use as individual filaments, fibres, yarns, woven fabrics, knitted fabrics or fleeces with thin carbon fibres by etching away during the carbonisation process, with the etching away of the fibres taking place uniformly, so that the cross-section of the fibres is appropriately reduced.

It is known to manufacture carbon fibres-starting from polyacrylonitrile fibres-in two steps, the organic fibres being exposed, in the first step, to a heat treatment between about 200 and 400 C. in an oxygen-containing atmosphere, and being carbonised, in the second step, in a flowing atmosphere containing hydrogen.

In most cases the heat treatment is carried out in air. In general, a tensile stress is applied to the fibres during the heat treatment. The carbonisation is preferably carried out in flowing hydrogen, at times also in flowing inert gas or in a flowing gas mixture consisting of hydrogen and inert gases. During the carbonisation, the temperature is in general increased by less than 300 C./ hour, up to the region of 1000 C. In some cases, the temperature is raised to between about 1500 and 1600" C. The carbonisation can be followed, in a third step, by a graphitisation at temperatures of up to- 3000 C., in inert gases. The two steps heat treatment and carbonisation are in most cases carried out in two separate temperature stages or heating zones. This is standard in continuous processes.

The finest-gauge spun fibres which can at the present time be manufactured on an industrial scale from acrylonitrile polymers have a gauge of about 1.5 dtex (decitex which is g./ 10,000 m.). This corresponds, for circular fibres, to a diameter of about 13 ,u.m. It has been found that fine-gauge polyacrylonitrile fibres are particularly suitable for the manufacture of carbon fibres. The danger of fibres fracturing when handling such materials is relatively great, especially if it is a matter of handling strips consisting of several thousand individual filaments. It is known that during the heat treatment in an oxygen-containing atmosphere and during the carbonisation a shrinkage of the fibres occurs, which amounts to a reduction in the fibre diameter of a total of about 30 to 50%. Carbon fibres can for practical purposes no longer be stretched. An elongation of 2% leads, in almost all cases, to the fibre tearing. Thus, practically no reduction in the fibre diameter is achievable by stretching the carbon fibre. For several applications, there is great interest in very thin carbon fibres, for example for the manufacture of carbon fibre-reinforced plastics, wherein the effectiveness of the reinforcing fibres increases with the ratio of length to diameter. Thin fibres are also particularly suitable for the manufacture of fibre materials for high temperature insulations in the form of carbon fleeces and random fibre waddings.

The present invention was based on the task of discovering a process for the manufacture of thin carbon fibres by etching thicker fibres away as uniformly as possible.

A process for the manufacture of fibre products with thin carbon fibres has now been discovered which starts from polyacrylonitrile fibres which are converted to carbon fibres in two steps, wherein, in the first step, a heat treatment in an oxygen-containing atmosphere at between and 400 C. takes place and in the second step the carbonisation takes place at temperatures at over 400 C., and the etching away during the carbonisation is carried out in a moving atmosphere containing water vapour, the amount of water vapour being 0.5 to 20% by volume of the moving atmosphere.

A preferred embodiment of the process according to the invention consists of the Water vapour concentration of the flowing atmosphere during the carbonisation being established by transporting the fibres, after the heat treatment and before the carbonisation, through a liquid which consists of water or contains water.

The special advantages of the process according to the invention consist of the fact that during the carbonisation an astonishingly uniform etching away of the fibres is achieved in a very simple manner and thin carbon fibres are obtained which have a cross-section which is proportionately reduced relative to that obtained without etching away. In this way, carbon fibres with a fraction of the cross-section obtained without etching away can be manufactured. In some cases it is possible to reduce the cross-section to a fifth. It is particularly important that the degree of reduction of the cross-section can be predetermined and controlled through the water vapour partial pressure in the carbonisation furnace and through the dwell time of the fibres in the carbonisation furnace. This is achieved in a particularly simple and constant manner by passing the fibres through a liquid containing water, with the water content of the liquid determining the water vapour partial pressure in the carbonisation furnace. It is also possible to control the water vapour partial pressure via the temperature of the liquid, and hence also via its viscosity.

A further advantage of the process according to the invention resides in that the manufacture of thin carbon fibres by etching away can be carried out continuously. Here the procedure followed is advantageously that after the heat treatment and before the carbonisation the fibres are continuously drawn through a liquid which consists of water or contains water. Herein, the liquid is then so arranged that it provides a gas-tight barrier at the inlet of the fibres into the carbonisation furnace. The use of a liquid to provide a gas-tight seal, and apparatus therefor, is claimed in our application Ser. No. 123,369, filed Mar. 11, 1971.

The process is carried out by, for example, drawing a ribbon of endless fibres of acrylonitrile polymers through a furnace in which a heat treatment in air lasting some hours, at temperatures between about 200 and 400 C., is carried out under tension. From this furnace, the heattreated fibres are then drawn into the carbonisation furnace, which is for example a tubular furnace. In this furnace, a temperature distribution which rises strictly continuously is maintained along the tube axis between the inlet and outlet, so that during their passage through this furnace the fibres experience a constantly increasing temperature between about room temperature and, for example, 1100 C. The total dwell time in the carbonisation furnace is between about 3 and 50 hours. The carbonisation is preferably carried out in flowing hydrogen, to which about 0.5 to volume percent of water vapour are admixed for simultaneous etching away of the fibres during the carbonisation. This gas mixture is advantageously blown into the carbonisation furnace on the same side as that where the fibres are drawn into the furnace. Inert gases can additionally also be admixed to the hydrogen-water vapour mixture.

Since it is on the one hand advisable to use not a very narrow inlet orifice for the fibres into the carbonisation furnace, whilst on the other hand it is desirable to manage with as economical a consumption of gas as possible without running the danger of air penetrating into the furnace, it has proved advantageous to predetermine and control the water vapour content of the flowing atmosphere in the carbonisation furnace by drawing the fibres, after the heat treatment and before enterin the carbonisation furnace, through a trough which is filled with water or a liquid containing water, with the liquid simultaneously serving as a gas-tight closure of the inlet orifice for the fibres into the carbonisation furnace.

The liquid is contained in a trough which is closed gastight by means of a lid. The lid possesses an orifice through which the fibres are passed into the liquid. The orifice is matched to the fibre product, namely individual fibre or filament, fibre strip consisting of many thousand individual fibres, yarn, woven fabric, knitted fabric or fleece. It should not be unnecessarily large, so as to keep the evaporation of the liquid low. A tube attachment, which is mounted gastight at the inlet into the carbonisation furnace, projects into the liquid from above. The fibre material is passed by means of direction-changing rollers or guide rollers shrough the liquid into the tube attachment and into the carbonisation furnace. The arrangement of the trough and of the tube attachment are so chosen that the liquid cannot run out of the trough into the furnace.

When carbonising filaments, fibres, fibre strips or yarns, it is preferred to use circularly cylindrical funnels for introducing this fibre material through the lid of the trough into the liquid and circularly cylindrical tube attachments for introducing the material into the carbonisation furnace. Rectangular slit orifices are used for plane structurues such as woven fabrics, fleeces and the like.

Possible liquids are water and all liquid substances containing water which release water vapour at room temperature or at the temperatures used for the carbonisation. In general, the manufacture of thin carbon fibres by etching away will not be combined with simultaneous dosing of the fibres with elements which are not contained in the fibre starting material. Liquids containing water which are particularly suitable for the process according to the invention are those which only contain elements from the class H, C, N and O, for example acids, alkalies, alcohols, aldehydes, ketones or hydrazine or hydrazine compounds. Hydrazine hydrate with hydrazine concentrations of between about 15 and 64% of N H is very suitable. In general, the procedure according to the invention will employ a liquid under normal pressure. The temperature of the liquid is below 100 C., preferably between about 0 and about C. It is advisable to use a transparent trough (for example of thick-walled glass or plastic) and transparent liquids, so that the fibre material can be observed at any time during its travel through the trough.

The amount of liquid which passes into the carbonisation furnace depends on the following parameters:

What passes into the furnace as vapour only depends on the vapour pressure of the liquid at the predetermined liquid temperature and on the internal crosssection of the tube attachment which dips into the liquid.

What passes into the furnace as liquid depends above all on the wetting of the heat-treated fibres by the liquid.

In the case of good wetting, the amount carried away depends on the viscosity of the liquid and on the specific surface area of the heat-treated fibres. The higher is the viscosity and the greater is the fibre surface, the more liquid is carried away into the furnace. The surface of the fibres depends on the starting material and its geometry, and also on the heat treatment in the oxygen-containing atmosphere and is, in the BET determination, generally of the order of magnitude of m. /g.

In the case of aqueous liquids where a change in the composition of the liquid occurs in the course of time as a result of different vapour pressures of the individual component and/ or selective wetting of the individual components on the heat-treated fibres, replenishment of the components in which the liquid would otherwise become depleted must be provided. This can for example be done by allowing the components in question to run dropwise into the liquid.

The figure shows a device for carrying out the process. The fibre product, consisting of an acrylonitrile polymer, for example a fibre strip, is drawn from the stock roll 1 possessing the brake drum 2 via the rollers 4 with the aid of the first pair of tension rollers 3 and the second pair of tension rollers 5, through the tubular furnace 6 possessing the quartz tube 7 and the funnels 8. A defined amount of air is passed into the tube 7 via the regulator 8a. At the lower roller of the second pair of tension rollers 5 there is a knife 11. The fibre strip is passed via the direction-changing roller 10 and through the funnel 18 into the trough 15 having a lid 16, and is passed via the direction-changing rollers 19, the dip tube 20 and the direction-changing rod 21 through the liquid 17 into the tubular furnace 12 having a quartz tube 13. In the quartz tube 13, in which the carbonisation takes place, an oxygen-free gas is introduced via the regulator 12a. The carbon fibre band is wound up on the roll 14.

The invention is explained yet further in the examples which follow:

EXAMPLE 1 An endless fibre strip consisting of 6000 individual fibres of acrylonitrile homopolymer, each of 1.8-1.9 dtex. is located on a stock roll 1, the avis of which is borne horizontally so that it can rotate. A brake drum 2 with variable adjustable braking action is seated on the axis. The fibre strip is continuously drawn from the stock roll between two vertical guide rods by means of a first pair of tension rollers 3 of which one roller is driven by a synchronous motor. Both rollers are cylindrical; their axes are arranged parallel to one another in a vertical plane. The driven, lower roller consists of mirror-polished high-quality alloy steel. The upper roller is rubber-coated, borne so that it is rotatable about its axis, and is Pressed with adjustable firmness'against the drive roller, with the axes of both rollers remaining parallel. Between the stock roll and the first pair of tension rollers there are four alternating rollers 4 borne in two different horizontal planes, which on drawing the strip from the brakes stock roll convert the almost circular strip cross-section into a rectangular cross-section of large width and small height.

A second pair of tension rollers 5 of exactly the same construction and the same nature, of which the lower drive roller is driven by a synchronous motor running at the same speed as the synchronous motor of the first pair of tension rollers, draws the fibre strip through an electrically heated tubular furnace 6 of cm. length. The heater winding of the furnace is so arranged that a constant temperature of 230 C. prevails over 80 cm. tube length and a gradual temperature rise of 4-5 C./cm. prevails at the furnace inlet side. A quartz tube 7 of 5 cm. internal diameter is located in the furnace. Glass funnels 8 are cemented into the tube at both ends, and these ensure that the strip is axially guided through the furnace. At the furnace inlet side, 50 1. (measured at N.T.P.) of air/hour are blown in via the regulator 8a,

and these together with the reaction vapours leave the furnace through the funnel orifice at the other end. The strip runs through the heat-treatment furnace at a speed of 17 cm./hour. The length of the strip is kept constant, whilst travelling through the heat-treatment furnace, by means of the two pairs of tension rollers at the inlet and outlet of the furnace.

The strip which leaves the heat-treatment furnace again has an almost circular cross-section, and this is converted by two rollers 9, borne in dilferent horizontal planes, into a rectangular cross-section of large width and small height, before the strip passes between the two tension rollers of the second pair. This results in uniform tension of the strip.

Behind the second pair of tension rollers, the strip is drawn vertically downwards via a direction-changing roller 10, in order to pass through the carbonisation furnace in a lower horizontal plane and in the converse direction (relative to the direction in the heat-treatment furnace).

To avoid difficulties which are caused by broken fibres, a knife 11-inclined at about 45 to the horizontal plane-is located with the cutting edge tightly against the lower roller above the level of the axis. The strip slides over the inclined knife plane.

The carbonisation occurs in a 300 cm. long tubular furnace 12 which possesses ten separate adjacent heater windings, each of about 30 cm. length (in the direction of the axis of the tube). At the ends, each of the ten windings are wound more tightly to avoid subsidiary minima in the temperature profile. Each winding is heated via its own variablev transformer, in such a way that a strictly monotonously rising temperature curve results along the axis of the tube between the inlet and outlet of the furnace. The furnace inlet is at 150 C. The maximum temperature, which is reached shortly before the furnace outlet, is 1050" C. A quartz tube 13 of 5 cm. internal diameter and smooth glazed internal wall lies in the furnace. The strip is drawn through this tube by means of a winding machine 14 with a cycling drive and slipping clutch.

The carbonisation takes place in a stream of hydrogen (100 1. measured at N.T.P. H /hour). The hydrogen is introduced via the regulator 12a at the same end of the furnace as the strip, but through a separate orifice. Excess hydrogen and reaction vapours leave the furnace at the other end, together with the carbonised strip.

A glass trough 15 with a lid 16 contains 2.5 l. of water as the liquid 17. Here, distilled water at room temperature is used. The strip is guided through a glass funnel 18 set in the lid 16 into the water and is drawn via two direction-changing rollers 19 of polished high quality alloy steel through a glass tube 20 of 8 mm. internal diameter and having rounded edges, which dips into the water, and over a glass rod 21 arranged horizontally at the level of the tubular furnace axis and at right angles to the axis, axially through the carbonisation furnace. The glass funnel 18 and the glass tube 20 are set in the lid 16 in a gas-tight manner; the lid is connected to the trough 15 in a gas-tight manner. The consumption of water is 2.4 ml./hour.

The carbon fibre strip manufactured in this way has a completely smooth, glossy appearance, and is very flexible and also suitable for textile applications. It has a C content of 97%. The average cross-section of the carbonised individual fibres is 20.10 cm. the specific resistance has a value of 17.10*S2.cm. The tensile strength was measured to be 1.5.10 kg. f./cm. and the modulus of elasticity 2.4.10 kg. f./cm.

If the same polyacrylonitrile fibre material is heattreated and carbonised in the same manner as described above, in the installation shown in FIG. 1, with the difference that items 15 to 19 (trough, lid, water and direction-changing rollers) and item 21 (glass rod) are omitted and the tube attachment 20 now has an internal diameter of 2.5 mm. and is arranged coaxially to the axis of the tubular furnace 12, a carbon fibre strip of similar appearance to that resulting on carbonisation under water vapour is obtained. The C content is 97%. However, the average cross-section of the carbonised individual fibres is 60.10- cm. and the specific electrical resistance has a value of 18.10- l'2.cm. The tensile strength was measured to be 1.4.10 'kg. f./cm. and the modulus of elasticity 2.3.10 kg. f./cm. (kg. f. is kilograms force or kiloponds) By carbonisation in an atmosphere containing water vapour it was thus possible to manufacture carbon fibres of one-third of the cross-section which is obtained without water vapour. It was possible to show by means of electron screen microscope photographs that the crosssectional shape of the carbon fibres is the same whether they are manufactured with or without water vapour. This means that etching away during carbonisation reduces the fibre cross-section proportionately; etching away takes place uniformly.

EXAMPLE 2 An endless fibre strip consisting of 3000 individual fibres of acrylonitrile homopolymer, each of 2.4 dtex, is heattreated and carbonised in the installation shown in FIG. 1. Here there are the following differences from the procedure described in Example 1: the three synchronous motors which drive the two pairs of tension rollers 3 and 5 and the Winding machine 14 were replaced by three synchronous motors which each run at twice the speeds compared to Example 1. In the heat-treatment furnace 6, the maximum temperature was raised to 250 C.; this temperature is constant over a tube length of 77 cm. (in Example 1, 230 C. is constant over 80 cm. tube length). The liquid 17 consists here of an 80% strength aqueous solution of hydrazine hydrate at room temperature. The heat treatment time and the carbonisation time are reduced to half relative to Example 1. The consumption of liquid 17 is here 1.9 ml./hour. All other conditions are exactly as has been described in Example 1.

The carbon fibre strip manufactured in this manner has a C content of 96.5%. The average cross-section of the individual fibre is 45.10- cm. and the specific electrical resistance is 20.10 9cm. The tensile strength has a value of 1.31.10 kg. f./cm. and the modulus of elasticity was found to be 2.7.10 kg. f./cm.

If the same fibre strip is heat-treated and carbonised in the manner described, except for the difference that items 15 to 19 and item 21 are omitted, and the tube attachment 20 now has an internal diameter of 2.0 mm. and is arranged in the direction of the axis of the tubular furnace 12, a carbon fibre strip having'the following properties is obtained:

An endless fibre strip consisting of 5000 untwisted individual fibres of acrylonitrile homopolymer, each of 1.5 to 1.6 dtex, is heat-treated and carbonised exactly as described in Example 2, with the sole difference that the liquid 17 consists of a mixture of acetic acid and water in the molecular ratio of 1:1, at room temperature. The consumption of liquid is 3.3 mL/hour.

A carbon fibre strip having the following properties is obtained:

C content percent 98 Average cross-section of the individual fibres cm. 46.10- Specific electrical resistance Q.cm 14.10- Tensile strength ....kg. f./cm. 1.9.10 Modulus of elasticity kg. f./cm. 2.9.10

7 If the carbon fibre strip is not passed through the liquid 17, that is to say is not etched away during the carbonisation, a carbon fibre strip having the following properties is obtained:

An endless fibre strip consisting of 8000 untwisted individual fibres of acrylonitrile homopolymer, each of 2.0 dtex, is heat-treated and carbonised as described in Example 2, but making the following changes: items 15 to 19 and item 21 are omitted. The tube attachment 20 here has an internal diameter of 2.5 mm. and is arranged coaxially to the axis of the tubular furnace 12. A mixture of hydrogen and water vapour is here blown in through the attachment sketched at right angles to the tubular axis of the furnace 12, 100 1. (measured at N.T.P.) of H /hour and 10 1. (measured at N.T.P.) of H vapour/ hour being used. For comparison, the carbonisation was also carried out without water vapour.

Carbon fibre strips having the following properties are obtained:

The invention also provides equipment for carrying out the process comprising the sequence of a stock roll, a pair of tension rollers at opposite ends of a tubular furnace, a trough with a lid for containing water or a water containing liquid, a further tubular furnace connected to the trough through a dip tube and the second tubular furnace being followed by a wind-up roll. Preferably a knife is mounted on the lower roller of the pair of tension rollers at the outlet end of the first tubular furnace.

What we claim is:

1. In a process for producing carbon fibre by heattreating polyacrylonitrile fibre in an oxidizing atmosphere at temperatures of from 150 to 400 C. followed by carbonising in a carbonising atmosphere at temperatures of over 400 C., the improvement comprising maintaining water vapour present in said carbonising atmosphere in amount of -20% by volume.

2. A process according to claim 1 wherein after said heat treatment and before the carbonisation the fibre is passed through an aqueous liquid, whereby the atmosphere in which the carbonisation is effected is provided with the water vapour.

3. A process according to claim 2 carried out as a continuous process.

4. A process according to claim 2 wherein the aqueous liquid is at a temperature below 100 C.

5. A process according to claim 4 wherein the aqueous liquid is at a temperature of between 0 and C.

6. A proces according to claim 5 wherein the aqueous liquid is under normal pressure.

7. A process according to claim 2 wherein the aqueous liquid does not contain compounds of elements other than hydrogen, carbon, nitrogen, and oxygen.

8. A process according to claim 7 wherein the aqueous liquid contains an acid, an alkali, an alcohol, an aldehyde, a ketone, a hydrazine or a hydrazine compound.

9. A process according to claim 8 wherein the aqueous liquid contains hydrazine hydrate with an hydrazine concentration of between 15 and 64% of N H 10. Process according to claim 1, wherein said polyacrylonitrile fibre has a gauge of 1.5 to 2.0 dtex.

11. Process according to claim 2, wherein said polyacrylonitrile fibre has a gauge of 1.5 to 2.0 dtex.

12. Process according to claim 2, wherein the carbonization is conducted in a furnace having an orifice inlet opening for the fibre, said aqueous liquid through which the fibre is passed forming a gas tight seal at said orifice inlet opening preventing penetration of air into the carbonizing furnace at the inlet orifice.

13. Process according to claim 12, wherein said polyacrylonitrile fibre has a gauge of 1.5 to 2.0 dtex.

14. Process according to claim 1, wherein said carbonising atmosphere is a moving atmosphere.

15. Process according to claim 14, wherein said carbonising atmosphere is a hydrogen-containing atmosphere.

16. Process according to claim 1, wherein the aqueous liquid contains acetic acid.

References Cited UNITED STATES PATENTS 3,552,923 1/ 1971 Carpenter et a1 23-209.1 3,337,301 8/1967 McWhorter et a1. 23209.1 3,053,775 9/1962 Abbott 252-421 EDWARD J. MEROS, Primary Examiner U.S. Cl. X.R. 264--29 UNITED STATES PATENT CERTIFICATE OF CORRECTION P N Dated Oetober 30, .1973

Inventor-(s) Roland Weisbeck, Gottfried Pampus Carlhans Sflling, Lothar Preis I v It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

1. C01. 6, line 72, "46.10 should be --28.10 cm Signed and sealed this '3rd day of- December" 1974 (SEAL) Attest:

McCOY M. GIBSON-JR. I y c; MARSHALL DANN Attesting Qfficer" Y Commissioner of; Patents FORM PC4050 (1M9) 

